The Pursuit of Hybrid Simulation Solutions for Ground Vehicles

Today’s vehicle developers are tasked with validating new technologies on vehicle platforms featuring numerous variants, often using fewer resources and in less time. While hybrid simulation – or the integration of physical tests and virtual models - holds tremendous potential for accelerating and enhancing vehicle development processes, the complexity of producing viable, laboratory-based solutions presents daunting technical challenges. In this two-part Force & Motion series, we examine the two implementations that MTS has pioneered to make the promise of hybrid simulation an automotive industry reality.

Mechanical Hardware-in-the-Loop (mHIL™)

Typically, vehicle component and subsystem testing occur in relative isolation; often, problematic interactions among components and subsystems are not brought to light until they manifest in vehicle prototypes on the proving ground, when they are difficult to pinpoint and expensive to resolve. Mechanical Hardware-in-the-Loop (mHIL™) testing, one of two implementations of hybrid simulation being pursued at MTS, represents an effective means to gain insight into these problems far earlier in the vehicle development process.

To date, hardware-in-the-loop testing combined virtual models with electronic hardware, such as power control units. Taking this concept a step further, MTS developed mHIL — an innovative technique that combines mechanical testing hardware and virtual models in a real time control loop.

With mHIL, a computer model (virtual simulation) of a vehicle draws low- to mid-frequency dynamics from a test rig applying mechanical loads to an actual component, such as a damper or suspension system. Data exchange occurs in real-time, enabling the vehicle model and the test rig to act on new data with each clock tick. In other words, the physical response of the component affects the behavior of the model and vice versa.

For OEMs under pressure to bring new vehicles to market faster, mHIL delivers important advantages. By allowing physical inputs from hard-to-model components and subsystems - and the subsequent simulation of their complex interactions with other vehicle systems - it generates high-fidelity vehicle, system and component behavior data much faster and more cost-effectively than traditional stand-alone testing or analysis. Validation and optimization of component and subsystem designs can occur earlier in the development process — with fewer and faster iterations — well before the first prototype hits the track.

Technical Challenges

While the concept of mHIL is fairly straightforward, executing it with accuracy and repeatability posed several technical challenges.

The first challenge involved the complexity of the vehicle models, which must undergo considerable adaptation to integrate physical components or subsystems. To overcome this challenge, MTS tasked its programming experts with exploring the variety of vehicle modeling architectures available, and developing the skills and techniques needed to work effectively within each to seamlessly replace virtual features with inputs from physical test rigs.

A second challenge was gaining optimal response from physical test rigs. To resolve this challenge MTS employed an array of special control techniques. One example of this is feed forward optimization, in which the control system anticipates future movements of the test rig and prepares the system to deliver force and motion at precisely the moment required by the real-time simulation.

The final challenge was achieving the necessary synchronization between models and the physical test rigs. Real-time, closed loop control requires a seamless flow of data between the two; commands must be sent, received and executed, and each step invites the risk of latency. To achieve dynamic control compensation throughout the mHIL test system, MTS developed a proprietary supervisor interface that handles logistics, timing and sequencing during initialization and test execution. The result was smooth data synchronization and stable, system-wide control. Without this interface, the model, test rig and controls would have to be programmed separately and fine-tuned during the test, and resulting test data would have to be synchronized off-line; all complex processes requiring the time and attention of already-stressed lab personnel.

First Solutions

MTS honed its approach to mHIL’s challenges through a series of pilot projects undertaken with major OEMs. One of the first was a four-corner damper system developed for Nissan to help them accelerate the development of an hydraulic body motion control (HBMC) system for a new vehicle platform —the first track trial was only six months away. The MTS mHIL solution replaced the vehicle model’s virtual dampers with four real ones in test rigs, allowing analysis at the component, subsystem and vehicle level. Capable of running literally thousands of test cases per month, this test bed enabled Nissan quickly detect irregular behavior in its HBMC system and correct the issue well in advance of prototype production.

MTS went on to develop even more complex mHIL solutions, including one for Hyundai used to validate the design of a new semi-active suspension system. The resulting quarter-suspension mHIL system allowed Hyundai Mobis to perform component and system validation on a single test bed without a full-vehicle prototype. Complete subsystem evaluation involved maneuver-based tests, fault and limit handling event testing and durability evaluation. This work revealed an unexpected interaction that caused an ECU fault. On the test track, this kind of interaction would have been extremely difficult to diagnose. With laboratory-based mHIL system, however, Hyundai engineers were able to easily isolate the root cause and refine the design accordingly.

Most recently, MTS worked with a major vehicle manufacturer to develop an mHIL steer system. Featuring a 5DOF test rig with ultra low torque measurement capabilities, this system is used to evaluate new electric power steering (EPS) system designs more quickly and easily, as well as establish better benchmarks and validation targets for suppliers of EPS subsystems. Its testing capabilities are vast, including steering rack durability and characterization, steering effort evaluation and EPS pre-tuning — all of which incorporate a real steering rack, column, I-shaft, tie rod ends, controls and wiring.

Pushing the Boundaries

The four-corner damper, quarter suspension and steer solutions represent only a fraction of the possible applications for mHIL technology. MTS engineers are currently working on mHIL systems for tires, brakes, roll bars and axles, as well as a configuration that incorporates kinematics and compliance deflection measurement (K&C) inputs. Future mHIL configurations may push the concept further by incorporating multiple subsystems (such as dampers, braking and steering) running in concert, all providing real-time data to the model. In theory, the ultimate mHIL system would enable all vehicle subsystems to run simultaneously.